1. Field of the Invention
This invention relates to an image-forming apparatus comprising an electron source and an image-forming member (fluorescent body) for forming an image by irradiation of electron beams emitted from the electron source as well as to a method of activating a getter in such an apparatus.
2. Related Background Art
In an image-forming apparatus comprising an electron source, a fluorescent body that operates as an image-forming member and fluoresces to form an image thereon when irradiated with electron beams emitted from the electron source and a vacuum container for containing the electron source and the image-forming member, the inside of the vacuum container has to be maintained at a high vacuum. Otherwise, any gas remaining in the vacuum container adversely affects the electron source so as to deteriorate the electron-emitting performance of the electron source and eventually make the apparatus unable to form a clear and bright image if the gas pressure rises significantly in the inside, although the extent of such an adverse effect depends on the type of the gas contained in the vacuum container. The gas in the vacuum container can be ionized by electron beams and the produced ions can be accelerated by the electric field being applied to the electrons from the electron source before some of the ions collide with the electron source to damage the latter. In some cases, the gas in the vacuum container can give rise to electric discharge in the inside and eventually destroy the image-forming apparatus.
The vacuum container of an image-forming apparatus is typically prepared by assembling glass components and bonding them with frit glass at the junctions thereof. The vacuum condition of the inside of the assembled and bonded vacuum container is maintained by means of a getter arranged within the vacuum container.
The getter in an ordinary CRT is a film of an alloy containing Ba as a principal ingredient and deposited on the inner wall of the container as the alloy is heated electrically or by means of a high frequency wave to evaporate. The deposited alloy adsorbs the gas produced within the container to maintain the inside to an enhanced degree of vacuum.
Meanwhile, there have been developed flat panel displays comprising an electron source realized by arranging a number of electron-emitting devices on a flat substrate. While the vacuum container of such a display apparatus has a volume smaller than that of a CRT, the surface area of the walls of the vacuum container of the display apparatus that can produce gas is not reduced if compared with a CRT. In other words, if the vacuum container of a flat panel display and that of a CRT produce gas to a same extent, the pressure rise in the container can be greater for the former than for the latter and the net result will be more catastrophic to the former. Additionally, while the vacuum container of a CRT has wall surfaces not carrying thereon any electron source or an image-forming member and a getter layer may be formed there, the inner surface area of the walls of the vacuum container of a flat panel display is mostly occupied by an electron source and an image-forming member. Any getter film formed on these components by vapor deposition can adversely affect the performance of the apparatus, giving rise to, for example, a short circuit wiring in it. Thus, the apparatus has a very limited area for forming a getter layer. While wall edges and corners of the vacuum container may be utilized for forming a getter film layer to make the image-forming member and the electron source (hereinafter collectively referred to as "the image display region") free from the gettering substance, such a measure can hardly provide a sufficiently large area for the getter that can satisfactorily adsorb the produced gas if the flat panel display has large dimensions.
In an attempt to solve the above problem and secure a large surface area for the getter, various proposals have been made, including the one according to which getter wires are arranged outside the image display region such as the peripheral wall of the display apparatus and a getter film is formed on the wall by vapor deposition in order to provide a sufficient surface area for the getter (Japanese Patent Application Laid-Open No. 5-151916 as schematically shown in cross section in FIG. 14A), one in which a getter chamber is annexed to a flat panel display in order to form a getter film (Japanese Patent Application Laid-Open No. 4-289640 etc. as schematically shown in FIG. 14B), and the one in which a space is provided between the electron source substrate and a rear plate of the vacuum container and a getter film is formed there (Japanese Patent Application Laid-Open No. 1-235152 etc.). With regard to the gas produced in the vacuum container of the flat panel display, there is a problem of local pressure rise in addition to the above-identified one. In an image-forming apparatus comprising an electron source and an image-forming member, gas is mainly produced within the vacuum container from the image-forming member to be irradiated with electron beams and the electron source itself. In a conventional CRT, the image-forming member and the electron source are separated from each other by a large distance and a getter film is formed on the walls of the vacuum container located between them so that the gas produced from the image-forming member disperses in different directions before it gets to the electron source and is partly absorbed by the getter film and, therefore, there occurs no significant pressure rise on the electron source. Additionally, since a getter film is also formed around the electron source itself, the gas discharged from the electron source itself does not accumulate remarkably to produce a significant local pressure rise there. In a flat panel display, to the contrary, the gas produced from the image-forming member can easily accumulate without satisfactorily dispersing to consequently give rise to a significant local pressure rise on the electron source because the image-forming member is located very close to the electron source. This pressure rise is more remarkable at the center than in the peripheral areas of the image display region because gas cannot disperse to get to the getter film. The produced gas can be ionized by electrons from the electron source, which can be accelerated by the electric field existing between the electron source and the image-forming member. Such ions can collide with the electron source to damage the latter and give rise to electric discharge in the inside to eventually destroy the electron source.
In an attempt to cope with this problem, there have been proposed flat panel displays in which a gettering substance is arranged within the image display region in order to immediately adsorb any gas generated in the inside. For instance, Japanese Patent Application Laid-Open No. 4-12436 discloses a method of forming gate electrodes of a gettering substance to be comprised in an electron source to extract electron beams. It describes a field emission type electron source using a conical projection for a cathode and a semiconductor electron source having in junctions. Japanese Patent Application Laid-Open No. 63-181248 discloses a method of arranging a gettering substance on control electrodes (including grid electrodes) to be comprised in a flat panel display in order to control electron beams within the display, said control electrodes being disposed between the cathodes and the face plate of the vacuum container of the display.
U.S. Pat. No. 5,453,659, "Anode Plate for Flat Panel Display having Integrated Getter", issued Sep. 26, 1995, to Wallace et al. discloses a gettering substance arranged in the gap between adjacent stripe-shaped fluorescent bodies in the display. According to the patent invention, the arranged gettering substance is electrically isolated from the fluorescent bodies and the electroconductive bodies electrically connected to the former in the display and the getter is activated by applying a voltage to it and irradiating and heating it with electrons emitted from the electron source of the display or, alternatively, by electrically energizing and heating it.
Needless to say, an electron-emitting device to be used for the electron source of a flat panel display desirably has a simple structure that can be manufactured by a simple method from the view point of technology and manufacturing cost. It is also desirable that such a device can be manufactured by laying thin films to a multilayer structure in a simple manner. A method for manufacturing a large flat panel display should desirably involve a technique such as printing that does not require the use of a vacuum apparatus.
From such a viewpoint, the method of forming gate electrodes of a gettering substance to be comprised in an electron source as disclosed in Japanese Patent Application Laid-Open No. 4-12436 listed above involves manufacturing steps of preparing conical cathode chips, forming junctions of semiconductors and other complicated operations to be carried out in a vacuum apparatus and is not adapted to manufacturing a large electron source because of the limitations relating to the manufacturing apparatus.
While the brightnesses of examples 1, 2, 4, 5, 6, and 7 are different from each other, they did not fall remarkably. The difference of brightness in the beginning is presumed to be caused by the difference of thickness of the getter layers, because, the number of electrons passing through a getter layer and reaching fluorescent bodies depends on the thickness of the getter layer.
In the cases of examples 3 and 8, while the efficacy is a little smaller than those of examples 1, 2, 4, 5, 6 and 7, the brightness drop is smaller than that of comparative example 1.
A display apparatus disclosed in Japanese Patent Application Laid-Open No. 63-181248 and comprising control electrodes disposed between the electron source and the face plate of the apparatus inevitably has a complicated configuration and requires cumbersome manufacturing steps for aligning such components.
U.S. Pat. No. 5,453,659 discloses a method of forming a gettering substance on an anode plate of an electron source. However, with this method, the gettering substance has to be electrically insulated from the fluorescent body of the electron source and patterning operations using a photolithography technique have to be repeated for precision processing.
The proposed method therefore involves cumbersome processing steps and the use of a large apparatus for photolithography so that the image-forming apparatuses manufactured by the method are inevitably subject to dimensional limitations.
An electron-emitting device that can meet the requirement of simple manufacturing method may be a lateral field emission type electron-emitting device or a surface conduction electron-emitting device. A lateral field emission type electron-emitting device comprises a cathode having a pointed electron-emitting region and a gate for applying a voltage to the cathode arranged on a flat substrate and is manufactured by means of a thin film deposition technique such as vapor deposition, sputtering or plating and an ordinary photolithography technique. A surface conduction electron-emitting device comprises an electroconductive thin film including a high electric resistance region and emits electrons when an electric current is made to flow therethrough. Such a device is disclosed in Japanese Patent Application Laid-Open No. 7-235255 applied by the application of the present patent application.
Since an electron source comprising surface conduction electron-emitting devices does not have gate electrodes having a configuration as described in Japanese Patent Application Laid-Open No. 4-12436, nor control electrodes as disclosed in Japanese Patent Application Laid-Open No. 63-181248, a getter cannot be arranged in the image display region of an image-forming apparatus provided with such an electron source, unlike the case of the above patent applications, and therefore should be disposed outside the image display region.
As described earlier, the image-forming member that is typically a fluorescent film subject to collisions with highly energized electrons and the electron source itself are the two greatest gas producers in an image-forming apparatus. If the pressure of the produced gas is relatively low, the gas can be adsorbed by the electron-emitting regions of the electron source to adversely affect the performance of the electron source and gas molecules that are ionized by electrons coming from the electron source are accelerated by the voltage applied between the image-forming member and the electron source or between the higher potential side and the lower potential side of the electron source and hit hard the higher potential side or the lower potential side of the electron source to damage it. If the gas pressure rises high locally and instantaneously, ions accelerated by the electric field collide with gas molecules to produce additional ions and consequently give rise to electric discharge within the apparatus. Then, the electron source can be locally destroyed to deteriorate the electron-emitting performance of the electron source. As for the image-forming member that is typically a fluorescent film, H.sub.2 O gas and other gas can be abruptly produced from the fluorescent body when electrons are emitted to cause the fluorescent body to fluoresce after the completion of the image-forming apparatus. Thus, the brightness of the image displayed on the display screen of the apparatus can become remarkably reduced in the initial stages of the operation of driving the apparatus. When the apparatus is driven further thereafter, gas can also be produced from areas around the electron source to gradually deteriorate the performance of the image-forming apparatus. So long as a getter is arranged outside the image display region of the apparatus as in the case of conventional image-forming apparatuses, the gas generated at the center of the image display region not only spend a long time before they get to the external getter region but can be adsorbed again by the electron source so that the getter cannot effectively operate to prevent the deterioration of the electron-emitting performance of electron source and the brightness of the image displayed on the image display region can be significantly degraded at the center. In view of the above identified drawbacks of conventional flat panel displays comprising not gate electrodes nor control electrodes, there is a demand for a novel flat panel display having an improved configuration wherein a gettering substance is arranged within the image display region so that any produced gas may be removed quickly and efficiently.